1. Field of the Invention
The present invention relates to power transformers having at least two concentric windings arranged around a core leg, and more particularly to such structure including means for preventing winding collapse or buckling.
2. Prior Art
Upon short-circuiting of a transformer of the above-mentioned type the windings are subjected to axial and radial short-circuit forces. The radial forces for the windings outside the main leakage field are always directed outwardly, and for the windings inside the main leakage field the radial forces are directed inwardly towards the core leg. The outwardly directed forces cause tensile stresses in the respective winding, which are normally easy to cope with. The forces which act radially inwardly cause more problems since they tend to reduce the diameter of the winding in question, and if the winding itself is not sufficiently stable or does not have sufficient support radially outwardly or inwardly, it will buckle. This may damage the insulation so that short-circuiting between the winding turns occurs. Windings having conductors of metal foil strip are particularly difficult to make short-circuit proof. Of course, it is possible to increase the stiffness of such a winding by gluing together the different turns, but in the case of large power transformers such a method involves manufacturing difficulties. Moreover, the types of glue which could be used in this connection do not possess fully satisfactory long-term electrical strength.
Several proposals have been made to arrange support inwardly of the inner winding, i.e. between the winding and the core. Because of the elasticity of the supporting material, this method cannot fully prevent buckling, but only restrict the extent of the buckling.